[Toybox] Add chsh command

[amichaelc]

This is my first patch to a mailing list, and my first email in general to a mailing list, so please tell me if I'm doing something wrong.

I added the `chsh` command, and I think it follows the style and whatnot pretty well. The lsb man page mentioned restricted accounts, but I can't find anything about those on Linux, so I didn't try to check for that in my code. Other than that, it follows lsb.

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